Electrochromic materials change color upon application of a voltage, generally a direct current (DC) voltage. This “color” change may be in the visible spectral region (about 400 to 700 nm) or in other regions, e.g. the near-infrared (IR) (about 0.7 to 2.0 microns), IR (about 2.0 to 45 microns), or microwave (1 mm to 1 m or 0.3 to 300 GHz). Electrochemical devices and materials have been used in rearview automobile mirrors, windows for buildings and flat panel displays. The change in color of an electrochromic material is generally due to a reduction/oxidation (“redox”) process within the material. Most electrochromic materials and devices are responsive in the visible spectral region. Examples of these include those based on metal oxides, such as WO3, MoO3 and Ni and Ta oxides, which generally change color from a dark state, e.g. dark blue, to a transparent state.
Conducting polymers are other examples of electrochromic materials. These are a relatively new class of electrochromic materials in which redox causes a change in conductivity as well as color. Redox in these materials is accompanied by inflow or outflow of counterions or “dopants” from the polymer matrix; the identity of the “dopant” may determine the type and intensity of the color change of the material.
Conducting polymers as used in electrochromic devices can be polymerized from their monomers using chemical or electrochemical polymerization. In the latter case the route to polymerization is generally through free radical ions of the monomer, and subsequently of oligomers, all generated electrochemically. In complications to electrochemical polymerization, larger oligomers may frequently precipitate out as salts of the dopants. Additionally, depending on the polymerization conditions, polymer regions with poor structure (e.g., substantial cross-linking) may also be created. Both the oligomer precipitates and the poorly structured polymer regions exhibit little or no redox activity (i.e., no electrochromic activity); they are essentially “dead” regions within the electrochromically active regions of the polymer. Such “dead regions” have a greater detrimental effect on the light state of conducting polymer electrochromic material than on its dark state, since they absorb light without switching or otherwise contributing to the electrochromism of the conducting polymer.
Electrochromic devices, which incorporate electrochromic materials, may contain the active electrochromic material in a transmissive-mode, wherein the light passing through the device is modulated, or in a reflective-mode, wherein the light reflected from the device is modulated. Reflective-mode devices are generally opaque. Electrochromic building windows are examples of transmissive-mode devices, whilst electrochromic rearview mirrors or flat panel displays are examples of reflective-mode devices.
Few electrochromics are capable of modulating IR light, i.e. altering the intensity and/or wavelength of light in the IR region; most electrochromics function in the Visible region. Certain conducting polymers are among the few materials capable of modulating light in both the Visible and IR regions. Those materials which are active in the IR region of the electromagnetic spectrum, capable of electrochromically modulating IR light may be referred to as “IR-active” electrochromic materials.
A relevant performance parameter or property for reflective-mode devices is the % Reflectance (% R), as a function of wavelength. A high % R in the reflective (high-reflectance) state and a low % R in the absorptive (low-reflectance) state leads to high contrast (Delta % R), which is an indicator of good performance. Other relevant performance parameters are switching time (between the low- and high-reflectance states), cyclability (number of switching cycles before appreciable degradation, generally indicated by greater than 5% degradation in Delta % R), and the broad-band (2.5 to 40 microns for the IR region) or narrow-band nature of the % R.
Another property relevant to the performance of reflectance-mode devices, specifically in the IR spectral region, is the emissivity; this is a property that describes the ability of a material to give out heat. Specifically, emissivity measures the ability of a tested material against the ability of a black body at the same temperature. This property varies from 0 to 1, with 0 being a non-emissive material and 1 being a highly emissive material. As (non-electrochromic) examples of emissive materials, white Teflon has low emissivity, whereas black carbon tape has high emissivity. Generally, but not always, emissivity=(1−reflectance). The parameter designated as emittance is integrated emissivity, generally over the region of IR wavelengths that are of thermal interest, i.e. about 2 microns to 40 microns. Sometimes the terms emissivity and emittance are, erroneously, confused and interchanged in the published literature.
Accordingly, a material exhibiting controllable IR electrochromism has the potential to vary its emittance in a controllable way. IR electrochromic devices that are variable emittance devices are needed in the field, which overcome the negative aspects of current electrochromic technologies.